An ultrasonic diagnostic apparatus is an apparatus which radiates ultrasonic pulses, which are generated from an ultrasonic transducer built into an ultrasonic probe, into a subject, receives reflected waves from a subject tissue or bloodstream by the ultrasonic transducer, and generates and displays image data, etc.
In this ultrasonic diagnostic apparatus, examples of index values, which are indicative of the effects of ultrasonic waves on a living body, include an MI (mechanical index) value, a TI (thermal index) value, and an Ispta (spatial-peak temporal-average intensity) value.
The MI value is an index value for evaluating the safety of a nonthermal action (mechanical action) of ultrasonic waves on a living body, and is defined as a value obtained by dividing a negative peak sound pressure by a square root of a central frequency. By the MI value, it is possible to express a generation process of cavitation which affects a living body tissue, such as a process in which a gas dissolved in a body fluid becomes bubbles due to a variation in pressure caused by ultrasonic waves.
The TI value is an index value for evaluating the safety of a thermal action of ultrasonic waves on a living body, and expresses the intensity of an ultrasonic output by a ratio thereof to an output of ultrasonic waves that is set at 1, by which the temperature of a living body tissue is raised by one degree by absorption attenuation of ultrasonic waves.
The Ispta value is a temporal average value of sound intensity at a point at which the sound intensity takes a maximum value in a sound field, and is defined as an intensity value of an ultrasonic output at a time when a value (pulse-intensity integral (PII)), which is obtained by time-integrating an instantaneous intensity of ultrasonic pulses, is averaged by a pulse-repetition period.
The recommendable maximum values of these index values are indicated by guidelines. Incidentally, the contents of the guidelines vary from country to country. For example, in the U.K., since a nonthermal action by ultrasonic waves is large in the scan of a fetus, eye and skull, a lower MI value is indicated as a recommendable maximum value (recommendable maximum MI value) for this scan than in the scan of other diagnosis regions. In addition, in the case of the U.K., in consideration of a thermal action by ultrasonic waves, a recommendable maximum scan time is indicated for each TI value. However, only for the purpose of research or only when necessary, is an examination permitted under the condition that a greater biological effect than under the recommended condition may be caused, with the responsibility being taken by the operator.
In some cases, the above-described index values relating to the ultrasonic output increase unintentionally due to a change of parameters such as a focus, a frequency, etc. during an examination.
Thus, in the ultrasonic diagnostic apparatus, there is a function for controlling the ultrasonic output so as to maintain a preset target MI value, even when parameters have been changed. In general, this function is used in a contrast-enhanced examination mode, in order to suppress the rupture of bubbles due to the effect of the mechanical action of ultrasonic waves. In addition, there is a function which makes it possible to select whether an ultrasonic output is to be controlled so as to maintain a preset target MI value or an ultrasonic output is to be controlled so as to maintain a target ultrasonic output, in accordance with the kind of diagnosis, for example, according to whether special diagnosis using a contrast medium is performed or not.
In the conventional ultrasonic diagnostic apparatus, there is no particular problem in usual cases. However, according to the inventor's study, there is room for improvement in that there is no function for setting maximum values of index values relating to ultrasonic outputs for individual scan regions such as a fetus, eye and skull.
Supplementarily, in the conventional ultrasonic diagnostic apparatus, there is room for improvement in that there is no function for suppressing the biological effect of ultrasonic waves to maximum values or less of index values for individual regions, and for maintaining safety, regardless of scan mode, such as a contrast-enhanced examination mode and other modes.
It is an object to provide an ultrasonic diagnostic apparatus which can suppress the biological effect of ultrasonic waves to the maximum value or less of the index value for each scan region, and can maintain safety, regardless of scan mode.